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1976 70 hp spark issue

J

JEguy

New member
Hello all, first post here and needing a bit of educating. Let me start by saying, I am by no means a marine mechanic of any type. I am a a boat lover and chronic tinkerer. I have a hobby of finding boats no one else wants and trying to breathe a little life back into them, by nature I spend more time getting old motors running than I do putzing about on the boat.

My current project is my personal fishing boat. The motor on it finally gave up and said no more. I had this 76 70hp than I had been slowly piecing together to one day hang on my boat. We got it hung and took it for a test run. It started well and idled but died when put into gear, I figured I had not been thorough on the low speed jets and started her back up. If I gave it hard full throttle she took off like a bullet and up the river we went.

I came home pulled the carbs and attended to the low speed jets, and headed back to the water. Now, she starts, idles (roughly though), and you can ease down the throttle and she push the boat at low speed, however, she doesn't make full power any more. I came home, checked over everything, even checked the timing (set by the Joe Reeves method), I am confident in my link and sync. My compression numbers are 1) 125, 2) 125, 3) 122. I have a strong blue spark that will jump a 7/16 gap.

It's almost like she has a misfire. We tried pulling plug wires to diagnose that and what we found is in the video below, I have never encountered this and don't know what to make of it, so I came here for help. Sorry for the long post, and any and all thoughts and suggestions are appreciated.

https://youtu.be/MhHh6-NnzBo
 
Your #1 coil looks like it needs to be replaced. Maybe do all 3 of them they look original. Do you have water on any of the plugs?
 
I did go ahead and order 3 new coils. I ordered the CDI coils, I would prefer the OEM version, but a budget is a budget and the CDI's we more affordable.

I do not have water on the plugs.
 
I have seen no evidence of water in the cylinders. the plugs really don't even looked fouled, they are all even and a nice tan color. I am waiting for the new coils to get here. I ordered them before I was sure that was the problem because that was all I could think of that would be the issue lol. Thank you all for the input and help. I have just never seen coils and plug leads behave that way.
 
I have the new coils on. The engine fired up well and seemed to idle a bit smoother. I have not had time to put it in the river to test it yet. I also found a factory service manual for this specific engine (70673) on ebay and I am waiting for it to arrive to confirm my link and sync. I don't have a ton of confidence in the way I have it set up now as I realized the instructions I was following were for a 1987 and newer engine.
 
Service manual arrived today, Finally, and was worth every penny of the $30 I paid for it. I now know that my link and sync is not correct and how to correct it. I also know that I need to set the idle timing and then the full advance timing. I will update running condition when these things are done. Hopefully this weekend.
 
Me too.
That link and sync was more important than I ever realized.
I got fooled because I did over a 73' 65HP many years ago and never had issues and did not do a link and sync. Just got lucky.

These guys (and $600 in parts) helped get my 76' 70HP running. Had it out yesterday as a matter of fact.
Hard to tell it isn't a new engine, but 45 years old. I swear it starts on 1-2 revolutions of the flywheel.
Has to be winterized now, and put up on the rack.
10-15.jpg

Jim
 
Last edited:
Update!!! Finally!!! 11/7/2021

Just got back in from a test run with my son. The factory service manual was a bargain at twice the price. Once we knew the proper procedure for the link and sync and how to get the base timing set correctly it was much easier to get the full advance timing set properly. With all of that done and the new coils she was starting and running better than ever on muffs and in a test tank. We took her out and gave it a shot. On the first run up the river she launched much better than she ever has but in about 15~20 seconds she was starting to act poorly again and he brought her back. I removed the hood to look at a few things and for the second run we just left it off. Result!! With the hood off she ran like a rocket again. When he came back I was watching it idle, and pandering why she ran perfect without the hood. It was then I noticed the stream off water coming down the side of the motor. I had him put the hood on an try again. I told him if it acted up again to remove the hood. he came back with the hood off. I think the coils were bad and causing a lot of the problem. I know my link and sync was nowhere near correct. I think my final issue is a leaking exhaust gasket under the power head that will get changed in a couple of weeks when I have time. All in all, I think this will be a good motor.

I really want to thank you all for the input and advice, I would still be scratching my head without the help I received.
 
Further Update.

Exhaust gasket got changed today. Not as bad as I thought it would have been. Cleaned up the midsection and pan while I have the power head off. Old gasket was shot, most of it was just debris in the cavity of the midsection. I will post another update when we get a test run done.
 
I've a little more work to do on mine too. Lower needs a reseal. Keep us updated.
These are becoming relics of the past. It is great to see them on the water!

Jim
 
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